1. Field of the Invention
The present invention relates to the field of offset printing. In particular, the present invention relates to a device for changing image plates in an offset printing press while the press is running.
2. Description of the Prior Art
Offset printing presses are well known in the prior art. In such devices, a blanket cylinder and an image or plate cylinder are used to print on one side of a web of material (e.g., paper) in contact with the blanket cylinder. The blanket cylinder receives an ink image from the image cylinder, and transfers that image to the web of material. The image cylinder usually contains an image plate, upon which is the image to be printed. The image cylinder alternatively can include a cassette of imageable material, which material is pre-imaged or imaged by an imager on the press. Various ink fountains and inker rolls, as well as dampeners, are used to transfer ink to the image plate or imageable material on the image cylinder, to thereby provide ink to the image cylinder for later transfer of the inked image to the blanket cylinder.
In offset printing presses of the prior art, each blanket cylinder is usually associated with a single image cylinder. In such printing presses, when it is time to change the image plate or cassette, either because of maintenance requirements or because of the need to print a new image, it is necessary to stop the press. Because a single image cylinder is used to transfer an image to the blanket cylinder, it is not possible to run the press while the image plate on, or cassette in, the image cylinder is being removed and replaced.
Stopping the press to change an image plate or cassette acts as an interruption to the printing process, slowing down the overall printing process. In addition to the delays caused by changing the image plate or cassette, there are also additional potential delays associated with restarting the press. These delays are the result of the dangers of breaking the web as the printing process is restarted and the press runs up to full speed. Starting and stopping of the press for image plate or cassette changes increases the risks of web breakage, which breakage can introduce additional delays by requiring the web to be re-threaded through the press and for any web scraps to be removed from the press.